1. Field of the Invention
The present invention relates to an ultrasonic motor and an electronic device having the ultrasonic motor, and more particularly to a positioning control of the ultrasonic motor.
2. Description of the Related Art
The ultrasonic motor is made up of: a vibrator which is attached with a piezoelectric body; a moving body; a pressurizing means that brings the moving body into a pressure contact with the vibrator; an electrode that applies a high frequency voltage to the piezoelectric body; and an oscillation driving circuit that generates the high frequency voltage, and frictionally drives the moving body by vibration waves that are generated in the vibrator due to the expansion/contraction motion of the piezoelectric body.
The ultrasonic motor has various characteristics such that the motor has a high torque at a low speed; has a large retaining torque in a non-energization state; is high in response and controllability; has no magnetic operation; is capable of being downsized and lightened in weight; and has extremely small operating sound. Therefore, the ultrasonic motor is available as a drive source of the positioning means of various electronic devices. For example, the ultrasonic motor can be used for driving, for example, an indicator device that controls the movement of a given angle, an angle control of the mirror, the driving of a pickup (head) of an information device, and so on.
Incidentally, the positioning of the ultrasonic motor is conducted by a control portion where a desired position is set and a positioning sensor that detects the present position of the moving body that is driven by the vibrator of the ultrasonic motor. In the case where the moving body is a rotor, the position sensor is made up of an encoder that is attached to the rotor. As shown in FIG. 4, the control portion always monitors a present-position signal from the position sensor and monitors whether the present position of the vibrator is the desired position, or not (201). In the case of detecting that the present position of the moving body reaches the desired position, the control portion checks whether overshooting occurs, or not (202). If the control portion judges that overshooting occurs, the control portion immediately reverses the driving of the ultrasonic motor (203). Also, if the control portion judges that overshooting does not occur, that is, if the control portion judges that the moving body comes to a stabilized state in which the moving body retains the desired position, the position correcting operation is completed.
As described above, in the conventional positioning control of the ultrasonic motor, the operation is immediately shifted to the reverse operation if overshooting occurs after the moving body has reached the desired value. The overshooting is an unavoidable phenomenon in the positioning control. Therefore, the reverse operation is repeatedly conducted. The amount of overshooting gradually decreases and finally converges on zero. That is, the ultrasonic motor becomes in a stabilized state. It is desirable that a period of time since the moving body first reaches the desired position until it becomes in the stabilized state, that is, a stabilization time is shorter. However, in the conventional positioning control of the ultrasonic motor which immediately shifts to the reverse operation for position correction, the stabilization time cannot be shortened, and therefore there arises such a problem that the positioning resolution is low.
A problem to be solved is to enhance the positioning resolution by shortening the stabilization time in the positioning control of the ultrasonic motor.
In order to solve the above-mentioned problem, according to the present invention, there is provided an ultrasonic motor comprising:
a vibrator to which a piezoelectric body is attached;
a moving body that is arranged so as to rotate clockwise or counterclockwise due to the vibration of the vibrator;
a pressurizing means that brings the moving body in pressure contact with the vibrator;
an electrode that applies a high frequency voltage to the piezoelectric body;
an oscillation drive circuit that generates the high frequency voltage;
a position sensor that generates a position signal of the moving body; and
a control portion that gives a command signal to the oscillation drive circuit and controls the supply of the high frequency voltage to the piezoelectric body to start/stop and rotate forward or backward the motor, in which feedback control is performed by the control portion so that the position of the moving body reaches a desired position while the position signal is compared with the desired position that is inputted in advance,
and the ultrasonic motor is characterized in that reverse operation is conducted after a stop command retaining time has elapsed since an instant of overshooting.
The stop command retaining time is set in the control portion in advance and is a time interval which is from a time since the control portion detects that the moving body reaches the desired position and issues a stop command until a time the control portion issues a start command.
The stop command retaining time is set in various ways in accordance with the state of the position control.
Therefore, the stop command retaining time is set to be substantially the same as a time interval which is from the time the control portion detects that the moving body reaches the desired position and issues a stop command until a time the moving body stops. Further, the stop command retaining time is set to be substantially the same as a time interval which is from the time the control portion detects that the moving body reaches the desired position and issues a stop command until a time the vibration of the vibrator stops. Furthermore, when the oscillation drive circuit constitutes a self-excitation oscillating circuit, the stop command retaining time is set to be the same as or longer than a time interval which is from the time the control portion detects that the moving body reaches the desired position and issues a stop command until a time the self-excitation oscillation of the self-excitation oscillating circuit stops.
The stop command retaining time may include a plurality of stop command retaining times that are different in time interval.
The stop command retaining time is set to be shorter in time interval as the amount of movement of the moving body to the desired position is smaller.
The stop command retaining time is set to be shorter as the speed of the moving body immediately before the control portion issues the stop command is slower.
Further, in an electronic device including an ultrasonic motor for solving the above-mentioned problem, the ultrasonic motor is constituted as described on the above.